This invention is related to electrical testing equipment and, more particularly, to an apparatus and method for independently measuring the resistance of a wound coil, and the resistance of the connection between the wound coil and a commutator segment on an armature of an electric motor.
With a commutator motor, it is desirable to test the wound coil resistance, and the connection resistance between the wound coils and commutator segments on the armature to assure that the armature is correctly wound, and to detect any wire breakage, short circuits or other faults.
When the coils are wound on the armature, part of each coil is looped around a small hook raised at an angle from the commutator, in order to connect the coil to the commutator segment. Spot welding or fusing is applied to the coil and hook in order to connect the coil and hook to the commutator. The welding or fusing process also burns the insulation coating off of the coil to obtain a close connection between the coil and commutator, thereby enabling a free flow of current.
If the spot welding or fusing is poorly performed, the hooks bent to the commutator segments may be forced to open outwardly by the centrifugal forces developed when the armature revolves at a high speed inside of the stator. If the hooks open, the coil wires may become loose from the commutator and disrupt the operation of the motor. Therefore, to avoid loose coil wires and the attendant problems, it is important to assure that each coil is correctly and securely fused to the commutator after the armature is fully wound and assembled. The fused connections can be tested by measuring the connection resistance between the wound coils and commutator segments. If the measured resistance exceeds a tolerance range, it is an indication that the connection may be faulty. In addition, the resistance of each wound coil can be measured, independently of the connection resistance, in order to verify that the coil is correctly wound.
In the past, it has been difficult to precisely measure the resistance of a single fused connection without cutting the winding, since the wound coils share a common connection with the commutator segments. Further, the connection resistance between the coils and commutator segments is typically small, on the order of 1% to 3% of the coil resistance. Therefore, measuring the voltage across a pair of commutator segments to which the tangs of one coil are connected can miss faulty fused connections, since the resistance produced by a faulty connection may be too small to fall outside the tolerance range for the coil resistance. Therefore, to detect connection faults it is desirable to measure the connection resistance directly, independent of the wound coil resistance.
Accordingly, there is a need for an apparatus and method for accurately measuring the connection resistance between a wound coil and a commutator segment on the armature of an electric motor, and for measuring the resistance of the wound coil independent of the connection resistance.